In that case, personally, I would go for sub 7 metres.
I can explain what we did (until wife's health changed our life style), if that helps, but, we are all different, our idea of "fun" may be your idea of hell, plus, some things are better afterwards, than at the time they are actually happening..
Typical routine was:
After Christmas (wife insisted on being home for Christmas, with family), ie January or possibly February, we would set off for Spain or Portugal (nothing booked, except outward channel crossing). We live near Newcastle-upon-Tyne, so, Dover is 350 mile drive approximately.
We saved Tesco vouchers, to get a discounted tunnel crossing, if not, we would take Dover-Dunkirk DFDS Ferry.
To get us there, we would plan 300-350 miles per day.
Bearing in mind the 350 mile drive to Dover, we would book a late evening crossing, with a view to arrive in France, approximately 22:00 - 23:00 French time. We would then drive South for about an hour, possibly two, to one of a number of "safe" Aires we knew of, stop, and sleep. This first stop would get us as far South as Abbeville, or, Baie de Somme, or Rouen, or even Neaufchatel-en-Bray.
I would explain here, go on any motorhome / camper van forum, and, there will be heated discussions about the use of aires/sites etc (they are typically about as calm and resigned as helmet debates on a cycling forum). Our take was, if we were just going to overnight, and Aire was fine, but, we didn't want to spend two weeks in a carpark, even if it was free or cheap. There are alll kinds of tales of robbery etc, we never encountered anything like that, but, we did exercise what I would call "common sense" ie just like in UK, if a place didn't "feel right", we didn't stay there.
Day two, we would be on the road 08:00, and head South, using none Toll roads (eg A28/A10/N10 typically), and heading more or less due south toward the Spanish Border. One of the reasons for using this route was weather, France can be remarkably cold in January, but, this route was typically snow free in the 15 years we used it. Depending on traffic, weather, how we felt, we would aim to cover 300-350 miles, which would put us around Poitiers or thereabouts.
Day three, if the indications were that snow was unlikely, we would head for Oloron-Sainte Marie, and the (free)
Somport tunnel, if, on the other hand, there was snow, or, it looked likely, we would keep West and head for San Sebastián. This decision was also tempered by if we were heading for Mediterranean or Portugal.
Day four, if heading for Mediterranean, we would aim to get there, typically Valencia area, if heading for Portugal, we would head toward Caceres.
There after, we would typically use sites, and stay at each for anything from 2 weeks to a month. We never found it necessary to book in advance (just watch out for holiday times like Easter, when sites do get full).
We typically stayed in Spain or Portugal until spring (say April/May), then, meander back through France, typically keeping more to the east, as the weather improved. Sometimes using on of the other (free) Pyrenees Tunnels (
Beilsa or
Vielha) to arrive, eventually at Calais at a very leisurely pace, to return home for the English "summer", then, in late August/September, we would "do" Germany, Hungary, Slovenia, Netherlands etc.
We did all that in a variety of 6.5-7metre motorhomes, sometimes towing a trailer, with motorcycle, and, always, carrying bicycles.
Generally speaking, it was great fun, but, there were some downers, like, the year we were heading South on our wedding anniversary (31st Jan), the weather was atrocious (monsoon like rain), We (well, I) took a wrong turning, the road was too narrow to turn around, with trailer, we had to get out, unhitch trailer (in pouring rain), turn van around, re-attach trailer. Then, when we arrived at our chosen stop (an Aire), it was under two metres of flood water, so, we had to spend the night in a nearby car park. My wife does occasionally remind me of that day, sometimes, she is smiling about it 😂